Selective cell-cell interactions are required to establish the precise three-dimensional architecture of the vertebrate nervous system through coordinated cell movements, regional partitioning, axon targeting and synapse formation. The cadherin superfamily comprises a large, diverse array of cell surface receptors that includes both the classical cadherins and the protocadherins. Both classical cadherins and protocadherins have been proposed to play roles in establishing neural circuitry in the developing brain. While the classical cadherins are known to be homophilic cell adhesion molecules, the involvement of protocadherins in cell adhesion remains uncertain. In our previous work, we showed that Protocadherin-19 (Pcdh19) associates with N-cadherin (Ncad) to form a heteromeric cis-complex. In preliminary data, we show that Pcdh19 mediates homophilic adhesion as part of the Pcdh19-Ncad complex, although it is not adhesive itself. The main hypothesis of this proposal is that protocadherins associate with classical cadherins to form cis-complexes with distinct adhesive properties. To investigate this hypothesis in more detail we propose the following Specific Aims. In Aim 1, we will test the hypothesis that members of the d-protocadherin family form cis-complexes with classical cadherins and determine whether these complexes mediate homophilic adhesion, as we have shown for Pcdh19-Ncad. In Aim 2, we will first determine the residues in Pcdh19 that constitute the adhesive interface, and then determine if this mechanism is general to Pcdh-Cdh complexes. Insights gained from these experiments will elucidate the molecular underpinnings of a novel mechanism of cell adhesion and will advance our knowledge of protocadherin and cadherin function, providing an essential foundation for understanding the roles of these molecules in neural development and elucidating the mechanisms that establish patterns of neural connectivity.